mg/L
Phosphate
What is it
Phosphate (PO4) is a plant macronutrient that enters through food, tap water, fertilizers and decaying organics. It is not a poison at normal planted-tank levels, but it reveals nutrient balance.
Why it matters
Plants need phosphate for energy transfer and growth. Too little can stall plants; too much combined with excess light, organics and poor CO2 can support algae.
Interactions with other parameters
Phosphate must be balanced with nitrate, potassium, trace elements, light and CO2. Looking at PO4 alone rarely explains algae.
Ideal ranges
| Tank type | Min | Max | Unit |
|---|---|---|---|
| Tropical community | 0.1 | 1.0 | mg/L |
| Planted high-tech | 1.0 | 3.0 | mg/L |
| Planted low-tech | 0.2 | 1.5 | mg/L |
| Shrimp tank | 0 | 0.5 | mg/L |
Out of range: what happens
Very low phosphate causes slow growth, dark leaves and green spot algae in planted tanks. Very high phosphate usually signals heavy feeding or dirty substrate and can contribute to nuisance algae when the tank is unstable.
Common Myths
- •Phosphate automatically causes algae; imbalance and excess organics are usually the real drivers.
- •Removing all phosphate helps plants; it can actually stop growth.
How to measure
Use a liquid PO4 test, especially before and after dosing fertilizers. Also test source water if phosphate rises even with light feeding.
How to adjust
Lower phosphate with water changes, substrate cleaning, reduced overfeeding and phosphate-removing media when needed. Raise it with a measured macro fertilizer for planted aquariums.
Pro Tips
Green spot algae on slow leaves often means phosphate is too low relative to light.
Rinse frozen foods or feed smaller portions if phosphate climbs quickly.
